The invention relates to a polishing pad which is useful for planarizing a semiconductor wafer or other substrate, and in particular, to a polishing pad of the type having multiple stacked layers.
xe2x80x9cMicroelectronic substratexe2x80x9d is intended to mean semiconductor devices or precursors thereto, including semiconductor wafers, semiconductor device layers comprising an insulator, semiconductor, barrier layer, conductor or any combination thereof.
Semiconductor wafers having integrated circuits fabricated thereon must be polished to provide a very smooth and flat wafer surface which in some cases may vary from a given plane by as little as a fraction of a micron. Such polishing is usually accomplished in a chemical-mechanical polishing (CMP) operation which utilizes a chemically active slurry that is buffed against the wafer surface by a polishing pad.
A polishing pad is often a relatively thin, disk-shaped article that is mounted on a platen of a polishing machine. Some polishing pads comprise two or more layers of different material that are coextensive and secured together by adhesive.
In the case of a two layer pad, the upper layer is a polishing layer that is relatively hard and stiff so that it maintains a planar polishing surface and provides a high rate of wafer material removal. The upper layer is substantially impermeable to the slurry and de-ionized water generally used in the polishing and washing operations.
The lower layer is typically a sub-pad that is softer than the upper layer to provide a cushion for the upper layer. The sub-pad tends to absorb the slurry and de-ionized water to which it is exposed. The sub-pad is shielded from these liquids by the coextensive upper layer and by the adhesive. However, the peripheral edge of the sub-pad is unshielded and is exposed to the liquid. As the liquid penetrates the sub-pad, physical properties of the sub-pad may change, thereby changing the cushioning effect of the sub-pad and the polishing performance of the stacked polishing pad.
Also, some polishing pads have a transparent window that permits the use of optical equipment for detecting a polishing endpoint. Liquid which penetrates into the sub-pad may reach the transparent window area and disturb the optical path through the window.
The present invention is directed to a stacked polishing pad comprising a polishing layer and a sub-layer. The polishing layer is substantially impermeable to water-based liquid (or is at least much less permeable than the sub-layer), while the sub-layer is liquid absorbent (or at least significantly more permeable to water-based liquids relative to the polishing layer). The polishing layer is preferably stacked on the sub-layer and is in contact therewith so as to shield an upper surface of the sub-layer from liquid contact. The sub-layer has an outer peripheral edge which is sealed to prevent liquid absorption into the sub-layer through the outer peripheral edge. When the stacked polishing pad is mounted on a platen of a polishing machine, the sub-layer has little, if any, exposed surface which can absorb liquid.
According to one embodiment of the invention, the stacked polishing pad may have an annular shape and include a central opening which is delineated by an inner peripheral edge. Preferably, the inner peripheral edge of the sub-layer is also sealed to prevent absorption of liquid into the sub-layer.
Peripheral edges of the sub-layer may be sealed by any suitable means including heat sealing, pressure embossing, and waterproof coating.